This invention relates to ceramic dielectric compositions and to multi-layer ceramic capacitors made therefrom, and more particularly this invention relates to improved dielectric ceramic compositions of the X7R variety, particularly suitable for producing low fire multi-layer ceramic capacitors which may have dielectric constants in excess of 3200.
There are a great number of prior U.S. patents which disclose various ceramic dielectric compositions that exhibit X7R characteristics--namely, compositions which meet the U.S. Electronics Industries Association (EIA) standard requiring a temperature coefficient of dielectric constant (K') that varies no more than about +15% from the reference value at 25.degree. C. over a temperature range of -55.degree. C. to 125.degree. C. Much of this wealth of prior art can be attributed to the fact that even small changes in the constituents and/or ratio of the constituents in dielectric compositions may generate new and unexpected characteristics. These new characteristics may produce enhanced properties when used in a multilayer ceramic capacitor.
For example, U.S. Pat. No. 4,816,430 discloses a composition having X7R characteristics, but it is necessary to fire products produced from such compositions at relatively high temperatures--namely, temperatures in excess of 1280.degree. C. As a consequence capacitors made from those compositions must utilize expensive electrode materials, such as for example pure palladium. My prior U.S. Pat. No. 5,128,289 also discloses dielectric ceramic materials having X7R characteristics, but they also constitute high fire compositions which must be sintered at temperatures which also must be in excess of 1280.degree. C. The last-mentioned compositions, however, have the advantage that they produce products having very dense, fine grained ceramic microstructures, thus permiting the use of much thinner dielectric layers during the production of multi-layer capacitors. Such capacitors in turn exhibit higher dielectric constants (K'), and thus permit the use of less expensive electrode materials for a given capacitance value.
The U.S. Pat. No. 4,540,676 and 5,296,426 disclose so-called low fire dielectric compositions having X7R characteristics, but although it is possible to fire capacitors made from such compositions in the range of 1100.degree. to 1150.degree. C., such capacitors do not necessarily exhibit dielectric constant values in excess of 3200.
It is an object of this invention, therefore, to produce dielectric ceramic compositions which exhibit X7R characterics, and which can be used to produce capacitors that are capable of developing dielectric constants (K') in excess of 3200.
Another object of this invention is to provide an improved X7R ceramic dielectric composition of the type described for producing a low fire (equal to or less than 1100.degree. C.) capacitor having a very high dielectric constant, low dissipation factor, and good temperature stability.
Still another object of this invention is to provide compositions of the type described which will produce multi-layer ceramic capacitors capable of generating dissipation factors which are equal to or less than 2.5% at 1KHz test frequency with a signal amplitude of 1.0 VRMS.
Other objects of the invention will be apparent hereinafter from the specification and from the recital of the appended claims.